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Full text of "Transmission Control Protocol"

Transmission Control Protocol 
Functional Specification 
JanUary 1980 
the local TCP. The buffer size between the two TCPs may be 
different .... 
The timeout, if present, permits the caller to set up a timeout 
for all buffers transmitted on the connection. If a buffer is 
not successfully delivered to the destination within the timeout 
period, the TCP will abort the connection. The present global 
default is 30 seconds. The buffer retransmission rate may vary; 
most likely, it will be related to the measured time for 
resDonees from the remote TCP. 
The TCP or some ccmDonent of the operating system will verify 
the users authority to open a connection with the specified 
precedence or security/compartment. The absence of precedence 
or security/compartment specification in the OPEN call indicates 
the default values should be used. 
TCP will accept incoming requests as matching only if the 
security/compartment information is exactly the same and only if 
the precedence is equal to or higher than the precedence 
requested in the OPEN call. 
The precedence for the connection is the higher of the values 
requested in the OPEN call and received from the incoming 
request, and fixed at that value for the life of the connection. 
Depending on the TCP implementation, either a local connection 
name will be returned to the user by the TCP, or the user will 
specify this local connection name (in which case another 
parameter is needed in the call). The local connection name can 
then be used as a short hand term for the connection defined by 
the <local socket, foreign socket> pair. 
Send 
Format: SEND(local connection name, buffer address, byte count, 
EOL flag, URGENT flag [, timeout]) 
This call causes the data contained in the indicaned user buffer 
to be sent on the indicated connection. If the connection has 
not been opened, the SEND is considered an error. Some 
implementations may allow users to SEND first; in which case, an 
automatic OPEN would be done. If the calling process is not 
authorized to use this connection, an error is returned. 
If the EOL flag is set, the data is the End Of a Letter, and the 
EOL bit will be set in the last TCP segment created from the 
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Transmissior, Control Protocol 
Functional Specification 
buffer. Tf the EOL flag is not set, subsequent SENDs will 
appear to be part of the same letter. 
Tf the URGENT flag is set, segments resulting from this call 
will have the urgent pointer set to indicate that some of the 
data associated with this call is urgent. This facility., for 
example, can be used to simulate "break" signals from terminals 
or error or completion codes from T/O devices. The semantics of 
this signal to the receiving process are unspecified. The 
receiving TCP will signal the urgent condition to the receivir, g 
process as long as the urgent pointer indicates that data 
preceding the urgent pointer has not been consumed by the 
receiving process. "ne purpose of urgent is to stimulate the 
receiver to accept some urgent data and to ir, dicate to the 
receiver when all the currently kno, urgent data has beer. 
received. 
The number of times the sending user's TCP signals urgent will 
not necessarily be equal to the number of times the receiving 
user will be notified of the presence of urgent data. 
if no foreign socket was specified in the OPEN, but the 
connection is established (e.g., because a LISTENing connection 
has become specific due to a foreign segment arriving for the 
local socket), then the designated buffer is sent to the implied 
foreign socket. i general, users who make use of OPEN with a 
unspecified foreign socket can make use of SEND without ever 
explicitly knowing the foreig socket address. 
However, if a SEND is attempted before the foreign socket 
becomes specified, a error will be returned. Users can use the 
STATUS call to determine the status of the connection. In some 
implementations the TCP may notify the user when an unspecified 
socket is bound. 
if a timeout is specified, the the current timeout for this 
connection is changed to the new one. 
I the simplest implementation, SEND would ot retur control to 
the sedig process until either the transmission was complete 
or the timeout had been exceeded. However, this simple method 
is both subject to deadlocks (for example, both sides of the 
connectio might try to do SENDs before doing ay RECEIVEs) and 
offers poor performance, so it is not recommended. A more 
sophisticated implementation would return immediately to allow 
the prscess to run concurrently with network I/O, and, 
furthermore, to allow multiple SENDs to be in progress. 
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Transmission Control Protocol 
Functional Specification 
January 1980 
Multiple SENDs are served in first come, first served order, so 
the TCP will queue those it cannot service immediately. 
We have implicitly assumed an asynchronous user interface in 
which a SEND later elicits some kind of SIGNAL or 
pseudo-interrupt from the serving TCP. An alternative is to 
return a response immediately. For instance, SENDs might return 
immediate local acknowledgmert, even if the segmert sent had not 
been acknowledged by the distant TCP. We could optimistically 
assume eventual success. If we are wrong, the connection will 
close anyway due to the timeout. in implementations of this 
kind (synchronous), there will still be some asynchronous 
sigrals, but these will deal with the connection itself, and not 
with specific segments or letters. 
NOTA BENE.' In order for the process to distinguish among error 
or success indications for different SENDs, it might be 
appropriate for the buffer address to be returned along with the 
coded response to the SEND request. ?CP-tc-user signals are 
discussed below, indicating the irformation which should be 
returned to the calling process. 
Receive 
Format: RECEIVE (local connection name, buffer address, byte 
count) 
This command allocates a receiving buffer associated with the 
specified connection. If no OPEN precedes this command or the 
calling process is not authorized to use this connection, an 
error is returned. 
In the simpIest implementation, control would not return to the 
calling program until either the buffer was filled, or some 
error occurred, but this scheme is highly subject to deadlocks. 
A more sophisticated implementation would permit several 
RECEiVEs to be outstanding at once. These would be filIed as, 
segments arrive. This strategy permits increased throughput at 
the cost of a more elaborate scheme (possibly asynchronous) to 
notify the calling program %hat a letter has been received or a 
buffer filled. 
If insufficient buffer space is given to reassemble a complete 
letter, the EOL flag will not be set in the response to the 
RECEIVE. The buffer will be filled with as much data as it can 
hold. The last buffer required to hold the letter is returned 
with EOL signaled. 
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Transmission Control Protocol 
Functional Specification 
The remaining parts of a partly delivered letter will be placed 
in buffers as they are made available via successive RECEIVEs. 
If a number of RECEIVEs are ou-standing, they may be filled with 
parts of a single long letter or with at most one letter each. 
The return codes associated with each RECEIVE will indicate what 
is contained in the buffer. 
If a buffer size was given in the OPEN call, then all buffers 
presented in RECEIVE calls must be of exactly that size, or an 
error indication will be returned. 
The URGENT flag will be set only if the receiving user has' 
previously been informed via a TCP-to-user signal, that urgent 
data is waiting. The receiving user should thus be in 
"read-fast" mode. If the URGENT flag is on, additional urgent 
data remains. If the URGENT flag is off, this call to RECEIVE 
has returned all the urgent data, and the user may now leave 
"read-fast" mode. 
To distinguish among several outstanding RECEIVEs and to take 
care of the case that a letter is smaller .than the buffer 
supplied, the return code is accompanied by both a buffer 
pointer and a byte count indicating the actual length of the 
letter received. 
Alternative implementations of RECEIVE might have the TCP 
allocate buffer storage, or the TCP might share a ring buffer 
with the user. Variations of this kind will produce obvious 
variation in user interface to the TCP. 
Close 
Format: CLOSE(local coznection name) 
This command causes the connection specified to be closed. If 
the connection is not open or the calling process is not 
authorized to use this connection, an error is returned. 
Closing connections is intended to be a graceful operation in 
the sense that outstanding SENDs will be transmitted (and 
retransmitted), as flow control permits, until all have been 
serviced. Thus, it should be acceptable 'to make several SEND 
calls, followed by a CLOSE, and expect all the data to be sent 
to the destination. It should also be clear that users should 
continue to RECEIVE on CLOSING connections, since the other side 
may be trying to transmit the Iast of its data. Thus, CLOSE 
means "I have no more to send" but does not mean "I will not 
receive any. more." It may happen (if the user level protocol is 
not well thought out) that the closing side is unable to get rid 
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Tran_mission Control Protocol 
Functional Specification 
January 1980 
of all its data before timing out. in this event, CLOSE turns 
into ABORT, and the closing TCP gives up. 
The user may CLOSE the connection at any time on his own 
initiative, or in response to various prompts from the TCP 
(e.g., remote close executed, transmission timeout exceeded, 
destination inaccessible). 
Because closing a connection requires communication with the 
foreign TCP, connections may remain in the closing state for a 
short time. Attempts to reopen the connection before the TCP 
replies to the CLOSE command will result in error responses. 
Close also mplies end of letter. 
Status 
Format: STATUS(local connection name) 
This is an implementstion dependent user command and could be 
excluded without adverse effect. Information returned would 
typically come from the TCB associated with the connection. 
This command returns a data block containing the following 
information: 
local socket, 
foreign socket, 
local connection name, 
receive window, 
send window, 
connection state, 
number of buffers awaiting acknowledgment, 
number of buffers pending receipt (including partial ones), 
receive buffer size, 
urgent state, 
precedence, 
security/compartment, 
and default transmission timeout. 
Depending on the state of the connection, or on the 
implementation itself, some of this information may not be 
available or meaningful. If the calling process is not 
authorized to use this connection, an error is returned. This 
prevents unauthorized processes from gaining information about a 
connection. 
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January 1980 
Transmission Control Protocol 
Functional Specification 
Abort 
Format: ABORT (local connection name) 
This co,nand causes all pending SENDs and RECEIVES to be 
aborted, the TCB to be removed, and a special RESET message to 
be sent to the TC? on the other side of the connection. 
Depending on the implementation, users may receive abort 
indications for each outstanding SEND or RECEIVE, or may simply 
receive an ABORT-acknowledgment. 
?C?-to-User Messages 
It is assumed that the operating system environment provides a 
means for the TCP to asychronously signal the user program. Whe 
the TCP does signal a user program, certain information is passed 
to the user. Often in the specification the information will be 
an error message. In other cases there will be information 
relating to the completion of processing a SEND or RECEIVE or 
other user call. 
The following information is provided: 
Local Conectio Name 
Response String 
Buffer Address 
Byte count (counts bytes received) 
End-of-Letter flag 
End-of-Urgent flag 
Always 
Always 
Send & Receive 
Receive 
Receive 
Receive 
TC?/Network Interface 
The TCP calls on a lower level protocol module to actually send and 
receive information over a network. One case is that of the ARPA 
intsrnetwork system where the lower level module is the Internet 
Protocol [2]. In most cases the following simple interfacewould be 
adequate. 
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Transmission Control Protocol 
Functional Specification 
January 1980 
The following two calls satisfy the requirements for the TCP to 
internet protocol module communication: 
SEND (dest, TOS, TTL, BufPTR, left, Td, DF, options => result) 
where: 
dest = destination address 
TOS = type of service 
TTL = time to live 
BufPTR = buffer pointer 
len = length of buffer 
Id = Identifier 
DF = Don't Fragment 
options = internet option data 
result = response 
OK = datagram sent ok 
Error = error in arguments or local network error 
Note that the precedence is included in the ?OS and the 
security/compartment is passed as an option. 
RECV (BufPTR => result, source, dest, prot, TOS, len) 
where: 
BufPTR = buffer pointer 
result = response 
OK = datagram received ok 
Error = error in arguments 
source = source address 
dest = destination address 
prot = protocol 
TOS = type of service 
options = internet option data 
let. = length of buffer 
Note that the precedence is .in the TOS, and the 
security/compartment is an option. 
When the TCP sends a segment, it executes the SEND call supplying 
all the arguments. The internet protocol module, on receiving 
this call, checks the arguments and prepares and sends the 
message. If the arguments are good and the segment is accepted by 
the local network, the call returns successfully. If either the 
arguments are bad, or the segment is not accepted by the Iocal 
network, the call returns unsuccessfully. On unsuccessful 
returns, a reasonable report should be made as to the cause of the 
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January 1980 
Trarsmission Control Protocol 
Functional Specification 
problem, but the details of such reports are up to individual 
implementations. 
When a segment arrives at the internet protocol modmle from the 
local network, either there is a pending RECV call from TCP or 
there is not. in the first case, the pending call is satisfied by 
passing the information from the segment to the TCP. in the 
second case, the TCP is notified of a pending segment. 
7he notification of a TCP may be via a pseudo interrupt or similar 
mechanism, as appropriate in the particular operating syste. 
environment of the implementation. 
A TCP's RECV call may then either be immediately satisfied by a 
pending segment, or the call may be pending until a segment 
art ives. 
We note that the internet Protocol provides arguments for a type 
of service and for a time to live. TEP uses the following 
settings for these parameters: 
Type of Service = Precedence: none, Package stream, 
Reliability: higher, Preference: speed, Speed: higher; or 
00011111. 
Time to Live = one minute, or 00111100. 
Note that the assumed maximum segment lifetime is two minutes. 
Here we explicitly ask that a segment be destroyed if it 
cannot be delivered by the internet system within one minute. 
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Transmission Control Protocol 
Functional Specification 
January 1980 
3.9. Event Processing 
The activity of the TCP can be charact_erized as responding to events. 
The events that occur can be cast into three categories: user calls, 
arriving segments, and timeouts. This section describes the 
processing the TCP does in response to each of the events. In many 
cases the processing required depends on the state of the connection. 
Events that occur: 
User Calls 
OPEN 
SEND 
RECEIVE 
CLOSE 
ABORT 
STATUS 
Arriving Segments 
SEGMENT ARRIVES 
Timeouts 
USER TIMEOUT 
RETRANSMISSION TIMEOUT 
The model of the TOP/user interface is that user commands receive an 
immediate return and possibly a delayed response via an event or 
pseudo interrupt. In the following descriptions, the term "signal" 
means cause a delayed response. 
Error responses are given as character strings. For example, user 
commands referencing connections that do not exist receive "error: 
connection not open". 
Please note in the following that all arithmetic on sequence numbers, 
acknowledgment numbers, windows, et cetera, is modulo 2**32 the size 
of the sequence number space. Also note that "=<" means less than or 
equal to. 
A natural way to think about processing incoming segments is to 
imagine that they are first tested for proper sequence number (i.e., 
that their contents lie in the range of the expected "receive window" 
in the sequence number space) and then that they are generally queued 
and processed in sequence number order. 
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January 1980 
Transmission Control Protocol 
Functional Specification 
When a segment overlaps other already received segments we reconstruct 
the segment to contain just the new data, and adjust the header fields 
to be consistent. 
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Transmission Control Protocol 
Functional Specification 
January 1980 
OPEN Call 
OPEN Call 
CLOSED STATE (i.e., TCB does not exist) 
Create a new transmission control block (TCB) to hold connection 
state information. Fill in local socket identifier, foreign 
socket, precedence, security/compartment, and user timeout 
information. Verify the security and precedence requested are 
allowed for this user, if not return "error: precedence not 
allowed" or "error: security/compartment not allowed." if active 
and the foreign socket is unspecified, return "error: foreign 
socket unspecified"; if active and the foreign socket is 
specified, issue a SYN segment. An initial send sequence number 
(iSS) is seIected and the TCP receive buffer size is selected (if 
'applicable). A SYN segment of the form <SEQ=ISS><CTL=SYN> is sent 
(this may include the buffer size option if applicable). Set 
SND.UNA to iSS, SND.NXT to ISS+I, SND.LBB to iSS+I, enter $YN-SENT 
state, and return. 
If the caller does not have access to the local socket specified, 
return "error: connection illegsl for this process". If there is 
no room to create a new connection, return "error: insufficient 
resources". 
LISTEN STATE 
SYN-SENT STATE 
SYN-RECEIVED STATE 
ESTABLISHED STATE 
FIN-WAIT-1 STATE 
FIN-WAIT-2 STATE 
TIME-WAiT STATE 
CLOSE-WAIT STATE 
CLOSING STATE 
Return "error: connection already exists". 
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January 1980 
SEND Call 
Transmission Control Protocol 
Functional Specification 
SEND Call 
CLOSED STATE (i.e., TCB does not e!st) 
If the user should no have access to such a connection, then 
return "error: connection illegal for this process". 
Otherwise, return "error: connection does not exist". 
LISTEN STATE 
If the foreign socket is specified, then change the connection 
from passive to active, select an ISS, and select the receive 
buffer size. Send a SYN segment, set SND.UNA to ISS, SND.NXT to 
ISS+I and SND.LBB to ISS+I. Enter SYN-SENT state. Data 
associated with SEND may be sent with SYN segment or queued for 
transmission after entering ESTABLISHED state. The urgent bit if 
requested in the command should be sent with the first data 
segment sent as a result of this command. If there is no room to 
queue the request, res?ond with "error: insufficient resources". 
If Foreign socket was not specified, then return "error: foreign 
socket unspecified". 
$YN-SENT STATE 
Queue for processing after the connection is ESTABLISHED. 
Typically, nothing can be sent yet, anyway, because the send 
window has not yet been set by the other side. If no space, 
return "error: insufficient resources". 
SYN-RECEIVED STATE 
Queue for later processing after entering ESTABLISHED state. If 
no space to queue, respond with "error: insufficient resources". 
ESTABLISHED STATE 
Segmentize the buffer, send or queue it for output, with a 
piggybacked acknowledgment (acknowledgment value = RCV.NXT) with 
the data. If there is insufficient space to remember this buffer, 
simply return "error: insufficient resources". 
If remote buffer size is not one octet, and, if this is the end of 
a letter, do the following end-of-Ietter/buffer-size adjustment 
processing: 
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Transmission Control Protocol 
Functional Specification 
January 1980 
SEND Call 
if EOL = 0 then 
SND.NXT <-SEG.SEQ + SEG.LN 
if EOL = 1 then 
While SND.LBB < SEG.SEQ + SEG.LEN 
Do SND.LBB <- SND.LBB + $ND.BS End 
SND.NXT <- SND.LBB 
If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the 
urgent pointer' in the outgoing segment. 
FIN-WATT-1 STATE 
F TN-WA TT-2 STATE 
TTME-WAI? STATE 
Return "error: connection closing" and do not service request. 
CLOSE-WAIT STATE 
Segmer. tize any text to be sent and queue for output. If there is 
insufficient space to remember the SEND, return "error: 
insufficient resources" 
CLOSING STATE 
Respond with "error: connection closing" 
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January 1980 
RECEIVE Call 
Transmissio Control Protocol 
Functional Specification 
RECEIVE Call 
CLOSED STATE (i.e., TCB does not exist) 
If the user should no have access to such a connection, return 
"error: connection illegal for this process". 
Otherwise return "error: connection does not exist". 
LISTEN STATE 
SYN-SENT STATE 
SYN-RECEiVED STATE 
Queue for processing after entering ESTABLISHED state. 
is no room to queue this request, respond with "error: 
insufficient resources". 
If there 
ESTABLISHED STATE 
If insufficient incoming segments are queued to satisfy the 
request, queue the request. If there is no queue space to 
remember the RECEIVE, respond with "error: insufficient 
resour ces". 
Reassemble queued incoming segments into receive buffer and return 
to user. Mark "end of letter ' (EOL) if this is the case. 
If RCV.UP is in advance of the data currently being passed to the 
user notify the user of the presence of urgent data. 
Wnen the TCP takes responsibility for delivering data to the user 
that fact must be communicated to the sender via an 
acknowledgment. The formation of such an acknowledgment is 
described below in the discussion of processing an incoming 
segment. 
F!N-WA!T-I STATE 
F IN-WA IT-2 STATE 
Reassemble and return a Ietter, or as much as will fit, in the 
user buffer. Queue the request if it cannot be serviced 
immediately. 
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Transmission Control Protocol 
Functional Specification 
January 1980 
RECEIVE Call 
TIME-WAIT STATE 
CLOSE-WAIT STATE 
Since the remote side has already sent FIN, RECEIVEs must be 
satisfied by text already reassembled, but not yet delivered to 
the user. If no reassembled segment text is awaiting delivery, 
the RECEIVE should get a "error: connection closing" response. 
Otherwise, any remaining text can be used to satisfy the RECEIVE. 
CLOSING STATE 
Return "error: connection closing" 
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January 1980 
CLOSE Call 
Transmission Control Protocol 
Functional Specification 
CLOSE Call 
CLOSED STATE (i.e., TCB does not--exist) 
if the user should no have access to such a connection, return 
"error: connection illegal for this process". 
Otherwise, return "error: connection does not exist". 
LISTEN STATE 
Any outstanding RECEIVEs should be returned with "error: closing" 
responses. Delete TCB, return "ok". 
SYN-$ENT STATE 
Delete the TCB and return "error: closing" responses to any 
queued SENDs, or RECEIVEs. 
SYN-RECEiVED STATE 
Queue for processing after entering ESTABLISHED state or 
segmentize and ser. d FZN segment. If the latter, enter FIN-WA!T-I 
state. 
ESTABLISHED STATE 
Queue this until all preceding SENDs have been segmentized, then 
form a FIN segment arid send it. Ir. any case, enter FiN-WAIT-1 
state. 
Y iN-WAiT- 1 STATE 
F IN-WAIT-2 STATE 
Strictly speaking, this is an error and should receive a "error: 
cornectior closing" response. An "ok" respor. se would be 
acceptable, too, as long as a second FIN is not emitted (the first 
FiN may be rerar, smitted though). 
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Functional Specification 
January 980 
CLOSE Call 
TIME-WAIT STATE 
Strictly speaking, this is an error and should receive a "error: 
connection closing" response. An "ok" response would be 
acceptable, too. However, since the FIN has been sent and 
acknowledged, nothing should be sent (or retransmitted). 
CLOSE-WAIT STATE 
Queue this request until all preceding SENDs have been 
segmentized: then send a FIN segment, enter CLOSING state. 
CLOSING STATE 
Respond with "error: connection closing" 
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January 1980 
ABORT Call 
Transmission Control Protocol 
Functional Specification 
ABORT Call 
CLOSED STATE (i.e., TCB does not exist) 
If the user should no have access to such a connection, return 
"error: connection illegal for this process". 
Otherwise return "error: connection does not exist". 
LISTEN STATE 
Any outstanding RECEIVEs should be returned with "error: 
connection reset" responses. Delete TCB, return "ok". 
SYN-SENT STATE 
Delete the TCB and return "reset" responses to any queued SENDs, 
or RECEIVEs. 
SYN-RECEIVED STATE 
Send a RST of the form: 
<SEQ=SND. NXT ><ACK=RCV. NXT ><CTL =RST, ACK> 
and return any unprocessed SENDs, or RECEIVEs with "reset" code, 
delete the TCB. 
ESTABLISHED STATE 
Send a reset segment: 
<SEQ=SND. NXT ><ACK =RCV. NXT> <CTL =RST, ACK > 
All queued SENDs and RECEIVEs should be given "reset" esponses; 
all segments queued for transmission (except for the RST formed 
above) or retransmission should be flushed, delete the TCB. 
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Transmission Control Protocol 
Functional Specification 
January 1980 
ABORT Call 
FIN-WAIT-1 STATE 
F IN-WA IT-2 STATE -- 
A reset segment (RS?) should be formed and sent: 
<SEQ =SND. NXT> <ACK=RCV. NXT> <C7L =RST, ACK > 
Outstanding SENDs, RECEiVEs, CLOSEs, and/or segments queued for 
retransmission, or segmentizing, should be flushed, with 
"connectior reset" notification to the user, delete the ?CB. 
TIME-WAiT S?A?E 
Respond with "ok" and delete the TCB. 
CLOSE-WAI? STATE 
Flush any pending SENDs and RECEiVEs, reurnirg "connectior reset" 
responses for them. Form and send a RS? segment: 
<SEQ=SND. NXT><ACK=RCV. NXT><CTL=RST, ACK> 
Flush all segment queues and delete the TCB. 
CLOSING STATE 
Respond with "ok" and delete the TCB; flush any remaining segment 
queues. if a CLOSE command is still pedirg, respored "error: 
connection reset". 
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STATUS Call 
Transmission Control Protocol 
Functional Specification 
STATUS Call 
CLOSED STATE (i.e., TCB does not ex-st) 
If the user should no have access to such a connection, return 
"error: connection illegal for this process". 
Otherwise return "error: connection does not exist". 
LISTEN STATE 
Return "state = LISTEN", and the TCB pointer. 
SYN-SENT STATE 
Return "state = SYN-SENT", and the TCB pointer. 
SYN-RECEIVED STATE 
Return "state = SYN-RECEiVED", and the TCB pointer. 
ESTABLISHED STATE 
Return "state = ESTABLISHED", and the TCB pointer. 
FiN-WAIT-1 STATE 
Return "state -- FIN-WAIT-l", and the TCB pointer. 
FIN-WAIT-2 STATE 
Return "state = FIN-WAIT-2", and the TCB pointer. 
TIME-WAiT STATE 
Return "state = TIME-WAIT and the TCB pointer. 
CLOSE-WAiT STATE 
Return "state = CLOSE-WAiT", and the TCB poirter. 
CLOSING STATE 
Returr "state = CLOSING", and the TCB pointer. 
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Transmission Control Protocol 
Functional Specification 
January 1980 
SEGMENT ARRIVES 
SEGMENT ARRIVES 
If the state is CLOSED (i.e., CB does not exist) then 
all data in the incoming segment is discarded. A incoming 
segment containing a RST is discarded. An incoming segment not 
containing a RST causes a RST to be sent in response. The 
acknowledgment and sequence field values are selected to make the 
reset sequence acceptable to the TCP that sent the offending 
segment. 
If the ACK bit is off, sequence number zero is used, 
<SEQ=O><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> 
If the ACK bit is on, 
<SEQ=SEG. ACK><C?L =RST> 
Return. 
If the state is LISTEN then 
first check for an ACK 
Any acknowledgment is bad if it arrives on a connection still in 
the LISTEN state. An acceptable reset segment should be formed 
for any arriving ACK-bearing segment, except another RST. The 
RST should be formatted as follows: 
<$E Q=SEG. ACK><CTL =RST> 
Return. 
An incoming RST should be ignored. Return. 
if there was no ACK then check for a SYN 
If the SYN bit is set, check the security. If the 
security/compartment on the incoming segment does not exactly 
match the security/compartment in the TCB then send a reset and 
return. If the SEG.PRC is less than the TCB. PRC then send a 
reset and return. If the SEG.PRC is greater than the TCB.PRC 
then set TCB.PRC<-SEG.PRC. Now RCV.NXT and RCV.LBB are set to 
SEG.SEQ+I, IRS is set to SEG.SEQ and any other control or text 
should be queued for processing later. ISS should be selected 
and a SYN segment sent of the form: 
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SEGMENT ARRIVES 
Transmission Control Protocol 
Functional Specification 
<SEQ=ISS ><ACK=RCV. NXT><CTL =SYN, ACK> 
SND.NXT and SND.LBB are set to ISS+I and SND.UNA to ISS. The 
connection state should be changed tc SYN-RECEIVED. Note that 
any other incoming control or data (combined with SYN) will be 
processed in the S'-RECEIVED state, but processing of SYN and 
ACK should not be repeated. If the listen was not fully 
specified (i.e., the foreign socket was not fully specified), 
then the unspecified fields should be filled in now. 
if there was no SYN but there was other text or control 
Any other control or text-bearing segment (not containing SYN) 
must have an ACK and thus would be discarded by the ACK 
processing. An incoming RST segment could not be valid, since 
it could not have been sent in response to anything sent by this 
incarnation of the connection. So you are unlikely to get here, 
but if you do, drop the segment, and return. 
If the state is SYN-SENT then 
first check for an ACK 
If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, or the 
security/compartment in the segment does not exactly match the 
security/compartment in the TCB, or the precedence in the 
segment is less than the precedence in the TCB, send a reset 
<SEQ =SEG. ACK > <CTL = RST> 
and discard the segment. Return. 
If SND.UNA =< SEG. ACK =< SND.NXT and the security/compartment 
and precedence are acceptable then the ACK is acceptable. 
SND.UNA should be advanced to equal SEG.ACK, and any segments on 
the retransmission queue which are thereby acknowledged should 
be removed. 
if the ACK is ok (or there is no ACK), check the RST bit 
If the RST bit is set then signal the user "error: connection 
reset", enter CLOSED state, drop the segment, delete TCB, and 
return. 
if the ACK is ok (or there is no ACK) and it was not a RST, check 
the SYN bit 
[Page 65] 
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Transmission Control Protocol 
Functional Specification 
January 1980 
SEGMENT ARRIVES 
If the $YN bit is on and e security/compartment and precedence 
are acceptable then, RCV.NXT and RCV.LBB are set to $EG.$EQ+I, 
IRS is set to $EG.SEQ. If SND.UNA > ISS (our SYN has been 
ACXed), change the connection state to ESTABLISHED, otherwise 
enter $YN-RECEIVED. In any case, form an ACK segment: 
<SE Q=SND. NXT ><ACK=RCV. NXT ><CTL =ACK> 
and send it. Data or controls which were queued for 
transmission may be included. 
If SEG.PRC is greater than TCB.PRC set TCB.PRC<-SEG.PRC. 
If there are other controls or text i the segment then continue 
processing at the fifth sep below where the URG bit is checked, 
otherwise return. 
[Page 66] 
------------------------------<page break>-----------------------------
January 1980 
SEGMENT ARRIVES 
Transmission Control Protocol 
Functional Specification 
Otherwise, 
first check sequence number 
$YN-RECEIVE D STATE 
ESTABLISHED STATE 
F IN-WAIT-1 STATE 
FIN-WA IT-2 STATE 
TIME-WAIT STATE 
CLOSE-WAIT STATE 
CLOSING STATE 
Segments are processed in sequence. Initial tests on arrival 
are used to discard old duplicates, but further processing is 
done in $EG.SEQ order. If a segment's contents straddle the 
boundary between old and new, only the new parts should be 
processed. 
There are four cases for the acceptability test for an incoming 
segment: 
Segment Receive Test 
Length Window 
0 0 
0 >0 
>0 0 
>0 >0 
SEG.SEQ = RCV.NXT 
RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND 
not acceptable 
RCV.NXT < $EG.$EQ+$EG.LEN =< RCV.NXT+RCV.WND 
Note that the test above guarantees that the last sequence 
number used by the segment lies in the receive-window. If the 
RCV.WND is zero, no segments will be acceptable, but special 
allowance should be made to accept valid ACKs, URGs and RSTs. 
If an incoming segment is not acceptable, an acknowledgment 
should be sent in reply: 
<SEQ =$ND. NXT> <ACX =RCV. N XT><CTL =ACK > 
If the incoming segment is unacceptable, drop it and return. 
[Page 57] 
------------------------------<page break>-----------------------------
Transmission Control Protocol 
Functional SpeCification 
January 1980 
SEGMENT ARRIVES 
second check security and precedence 
If the security/compartment and precedence in the segment do not 
exactly match the security/compartment and precedence in the .TCB 
then form a reset and return. 
Note this check is placed following the sequence check to prevent 
a segment from an old connection between these parts with a 
different security or precedence from causing an abort of the 
current connection. 
third check the ACK field, 
SYN-RECEIVED STATE 
If the RST bit is off and SND.UNA < SEG.ACK =< SND.NXT then set 
SND.UNA <- SEG. ACK, remove any acknowledged segments from the 
retransmission queue, and enter ESTABLISHED state. 
If the segment acknowledgment is not acceptable, form a reset 
segment, 
<SEQ=SEG. ACK><CTL =RST> 
and send it, unless the incoming segment is an RST (or there is 
no ACK), in which case, it should be discarded, then return. 
ESTABLISHED STATE 
If SND. UNA < SEG. ACK =< SND.NXT then, set SND.UNA <- SEG. ACK. 
Any segments on the retransmission queue which are thereby 
entirely acknowledged are removed. Users should receive 
positive acknowledgments for buffers which have been SENT and 
fully acknowledged (i.e., SEND buffer should be returned with 
"ok" response). If the ACK is a duplicate, it can be ignored. 
If the segment passes the sequence number and acknowledgment 
number tests, the send window should be updated. If 
SND.WL =< SEG.SEQ, set SND.WND <- SEG.WND and set 
SND.WL <- SEG.SEQ. 
If the remote buffer size is not one, then the 
end-of-le%ter/buffer-size adjustment to sequence numbers may 
have an effect on the next expected sequence number to be 
acknowledged. It is possible that the remote TC? will 
acknowledge with a SEG.ACK equal to a sequence number of an 
[Page 68 ] 
------------------------------<page break>-----------------------------
January 1980 
SEGMENT ARRIVES 
Transmission Control Protocol 
Functional Specification 
octet that was skipped over at the end of a letter. This a mild 
error on the remote TCPs part, but not cause for alarm. 
FIN-WAIT- 1 STATE 
FIN-WAIT-2 STATE 
In addition to the. processing for the ESTABLISHED state, if the 
retransmission queue is empty, the user's CLOSE can be 
acknowledged ("ok") but do not delete the TCB. 
TIME-WAIT STATE 
The only thing that can arrive in this state is a retransmission 
of the remote FIN. Acknowledge it, and restart the 2 MSL 
timeout. 
CLOSE-WAIT STATE 
Do the same processing as for the ESTABLISHED state. 
CLOSING STATE 
If the ACK acknowledges our FIN then delete the TCB (enter the 
CLOSED state), otherwise ignore the segment. 
fourth check the RST bit, 
SYN-RECEIVED STATE 
If the RST bit is set then, if the segment has passed sequence 
and acknowledgment tests, it is valid. If this connection was 
initiated with a passive OPEN (i.e., came from the LISTEN 
state), then return this connection to LISTEN state. The user 
need not be informed. If this connection was initiated with an 
active OPEN (i.e., came from SYN-SENT state) then the connection 
was refused, signal the user "connection refused". In either 
case, all segments on the retransmission queue should be 
removed. 
[Page 69] 
------------------------------<page break>-----------------------------
Transmission Control Protocol 
Functional Specification 
January 1980 
SEGMENT ARRIVES 
ESTABLISHED 
FIN-WAIT-1 
FINWAIT-2 
CLOSE-WAiT 
CLOSING STATE 
If the RST bit is set then, any outstanding RECEIVEs and SEND 
should receive "reset" responses. All segment queues should be 
flushed. Users should also receive an unsolicited general 
"connection reset" signal. Enter the CLOSED state, delete the 
TCB, and return. 
TIMEAIT 
Enter the CLOSED state, delete the TCB, and return. 
fifth, check the SYN bit, 
S-RECEIVED 
ESTABLISHED STATE 
If the S' bit is set, check the segment sequence number against 
the receive window. The segment sequence number must be i the 
receive windowl if not, ignore the segment. If the SYN is 
and SEG.SEQ = IRS then everything is ok and no action is needed; 
but if they are not equal, there is an error and a reset must be 
sent. 
If a reset must be sent it is formed as follows: 
<SEQ=SEG. ACK><CTL=RST> 
The connection must be aborted as if a RST had bee received. 
FIN-WAIT STATE- 
FIN-WAIT STATE-2 
TIME-WAIT STATE 
CLOSE-WAIT STATE 
CLOSING STATE 
This case should not occur, since a duplicate of the SYN which 
started the current connectio incarnatio will have bee 
filtered i the SEG.$EQ processing. Other SYN's will have bee 
rejected by this test as well (see $YN processing for 
ESTABLISHED state). 
[Page 70] 
------------------------------<page break>-----------------------------
January 1980 
SEGMENT ARRIVES 
Transmission Control Protocol 
Functional Specification 
sixth, check the URG bit, 
ESTABLISHED STATE 
FIN-WAIT-1 STATE 
FIN-WAIT-2 STATE 
If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and signal 
the user that the remote side has urgent data if the urgent 
pointer (RCV.UP) is in advance of the data consumed. If the 
user has already been signaled (or is still in the "urgent 
mode ') for this continuous sequence cf urgent data, dc not 
signal the user again. 
TIME-WAIT STATE 
CLOSE-WAIT STATE 
CLOSING 
This should not occur, since a FIN has been received from the 
remote side. Ignore the URG. 
seventh, process the segment text, 
ESTABLISHED STATE 
Once in the ESTABLISHED state, it is possible to deliver segment 
text to user RECEIVE buffers. Text from segments can be moved 
into buffers until either the buffer is full or the segment is 
empty. If the segment empties and carries an EOL flag, then the 
user is informed, when the buffer is returned, that an EOL has 
been received. 
If buffer size is not one octet, then do the following 
end-of-letter/buffer-size adjustment processing: 
if ECL = 0 then 
RCV. NXT <- SEG.SEQ + SEG. LEN 
if EOL = 1 then 
WZnile RCV.LBB < SEG.SEQ+SEG.LEN 
Do RCV.LBB <- RCV.LBB + RCV.BS End 
RCV. NXT <- RCV.LBB 
When the TCP takes responsibility for delivering the data to the 
user it must also acknowledge the receipt of the data. Send an 
acknowledgment of the form: 
[Page 71] 
------------------------------<page break>-----------------------------
Transmission Control Protocol 
Functional Specification 
January 1980 
SEGMENT ARRIVES 
<SEQ =S ND. N XT > <ACi( =RCV. NXT> <CTL =ACK > 
This acknowledgment should beiggybacked on a segment being 
transmitted if possible without incurring undue delay. 
FIN-WAIT-1 STATE 
F IN-WA IT-2 STATE 
If there are outstanding RECEIVEs, they should be satisfied, if 
possible', with the text of this segment; remaining text should 
be queued for further processing. If a RECEIVE is satisfied, 
the user should be notified, with "end-of-letter" (EOL) signal, 
if appropriate. 
TIME-WAIT STATE 
CLOSE-WAIT STATE 
This should not occur, since a FiN has been received from the 
remote side. Ignore the segment text. 
eighth, check the FiN bit, 
Send an acknowledgment for the FIN. Signal the user "connection 
closing", and return any pending RECEIVEs with same message. Note 
that FiN implies EOL for any segment text not yet delivered to the 
user. If the current state is ESTABLISHED, enter the CLOSE-WAIT 
state. If the current state is FIN-WAI?-I, enter the CLOSING 
state. If the current state is FIN-WAIT-2, enter the TIME-WAIT 
state. 
and return. 
[Page 72] 
------------------------------<page break>-----------------------------
January 1980 
USER TIMEOUT 
Transmission Control Protocol 
Functional Specification 
USER TIMEOUT 
For any state if the user timeout expires, flush all queues, signal 
the user "error: connection aborted due to user timeout" in general 
and for any outstanding calls, delete the TCB, and return. 
RETRANSMISSION TIMEOUT 
For any state if the retransmission timeout expires on a segment in 
the retransmission queue, send the segment at the front of the 
retransmission queue again, reinitialize the retransmission timer, 
and return. 
[Page 73] 
------------------------------<page break>-----------------------------
Transmission Control Protocol January 1980 
[Page 74 ] 
------------------------------<page break>-----------------------------
January 1980 
Transmission Control Protocol 
[Page 14 ] 
------------------------------<page break>-----------------------------
January 1980 
Transmission Control Protocol 
3. FUNCTIONAL SPECIFICATION 
3.1. Header Format 
TCP segments are sent as internedatagrams. The Internet. Protocol 
,header carries several information fields, including the source and 
destination host addresses [2]. A TCP header follows the internet 
header, supplying information specific to the TCP protocol. This 
division allows for the existence of host level protocols other than 
TCP. 
?CP Header Format 
0 1 2 3 
01234567890123456789012345678901 
Source Port 
Sequence Number 
Acknowledgment Number 
Data ' 
Offsetl Reserved ' 'CIO 
,Ri ISlYIII Window 
Checksum 
Options 
data 
TCP Header Format 
Note that one tick mark represents one bit position. 
Figure 3. 
Source Port: 16 bits 
The source port number. 
Destination Port: 16 bits 
The destination port number. 
[Page 15 ] 
------------------------------<page break>-----------------------------
Transmission Control Protocol 
Functional Specification 
January 1980 
Sequence Number: 32 bits 
The sequence number of the first dat-octet in this segment (except 
when SYN is present). 
Acknowledgment Number: 32 bits 
If the ACK control bit is set this field contains the value of the 
next sequence number the sender of the segment is expecting to 
receive. Once a connection is established this is always set. 
Data Offset:  bits 
?he number of 32 bit words in the ?CP Header. This indicates where 
the data begins. The ?CP header including options is an itegral 
number of 32 bits long. 
Reserved: 6 bits 
Reserved for future use. Must be zero. 
Control Bits: 8 bits (from left to right): 
URG: 
ACK: 
EOL: 
RST: 
$YN: 
FIN: 
Urgent Pointer field significant 
Acknowledgment field significant 
End of Letter 
Reset the connection 
Synchronize sequence numbers 
No more data from sender 
Window: 16 bits 
The number of data octets beginning with the one indicated in the 
acknowledgment field which the sender of this segment is willing to 
accept. 
Checksum: 16 bits 
The checksum field is the 16 bit one's complement of the one's 
complement sum of all 16 bit words in the header and text. If a 
segment contains an odd number of header and text octets to be 
checksummed, the last octet is padded on the right with zeros to 
form a 16 bit word for checksum purposes. /ne pad is not 
transmitted as part of the segment. While computing the checksum, 
the checksum field itself is replaced with zeros. 
The checksum also covers a 96 bit pseudo header conceptually 
prefixed to the TCP header. This pseudo header contains the Source 
[Page 16] 
------------------------------<page break>-----------------------------
January 19UU 
Transmission Control Protocol 
Functional Specification 
Address, the Destination Address, the Protocol, and TCP length. 
This gives the TCP protection against misrouted segments. This 
information is carried in the Internet Protocol and is transferred 
across the TCP/Network interface in the arguments or results of 
calls by the TCP on the IP. 
' Source Address 
! 
' Destination Address 
, zero I PTCL  TCP Length 
The TCP Length is the TCP header plus the data length in octets 
(this is not an explicitly transmitted quantity, but is computed 
from the total length, and the header length). 
Urgent Pointer: 16 bits 
This field communicates the current value of the urgent pointer as a 
positive offset from the sequence number in this segment. 
urgent pointer points to the sequence number of the octet following 
the urgent data. This field should only be interpreted in segments 
with the URG control bit set. 
Options: variable 
Options may occupy space at the end of' the TCP header and are a 
multiple of 8 bits in length. All options are included in the 
checksum. An option may begin on any octet boundary. There are two 
cases for the format of an option: 
Case 1: 
Case 2: 
A single octet of option-kind. 
An octet of option-kind, an octet of option-length, and 
the actual option-data octets. 
The option-length counts the two octets of option-kind and 
option-length as well as the option-data octetso 
Note that the list of options may be shorter than the data offset 
field might imply. The content of the header beyond the 
End-of-Option option should be header padding (i.e., zero). 
A TCP must implement all options. 
[Page 17] 
------------------------------<page break>-----------------------------
Transmission Control Protocol 
Functional Specification 
January 1980 
Currently defined options include (kind indicated in octal): 
Kind Length Meaning 
0 - End of option list. 
1 - No-Operation. 
100 - Reserved. 
105  Buffer Size. 
Specific Option Definitions 
End of Option List 
Iooooooooi 
Kind=O 
This option code indicates the end of the option list. This 
might not coincide with the end of the TCP header according to 
the Data Offset field. This is used at the end of all options, 
not the end of each option, and need only be used if the end of 
the options would not otherwise coincide with the end of the TCP 
header. 
No-Operation 
', ooooooo' ', 
,4 ',1" 
Eind=l 
This option code may be used between options, for example, to 
align the beginning of a subsequent option on a word boundary. 
There is no guarantee that senders will use this option, so 
receivers must be prepared to process options even if they do 
not begin on a word boundary. 
Buffer Size 
1010001011000001001 
Kind=105 Length=a 
buffer size 
[ Page 18 ] 
------------------------------<page break>-----------------------------
January 1980 
Transmission Control Protocol 
Functional Specification 
Buffer Size Option Data: 16 bits 
If this option is present,-then it communicates the receive 
buffer size at the TCP which sends this segment. This field 
should only be sent in the initial connection reque'st (.e., 
in segments with the SYN control bit set). If this option is 
not used, the default buffer size of one octet is assumed. 
Padding: variable 
The TCP header padding is used to ensure that the TCP header ends 
and data begins on a 32 bit boundary. The padding is composed of' 
zeros. 
3.2. Terminology 
Before we can discuss very much about the operation of the TCP we need 
to introduce some detailed terminology. The maintenance of a TCP 
connection requires the remembering of several variables. We conceive 
of these variables being stored in a connection record called a 
Transmission Control Block or TCB. Among the variables stored in the 
TCB are the local and remote socket numbers, the security and 
precedence of the connection, pointers to the user's send and receive 
buffers, pointers to the retransmit queue and to the current segment. 
In addition several variables relating to the send and receive 
sequence numbers are stored in the TCB. 
Send Sequence Variables 
SND.UNA - send unacknowledged 
SND.NXT - send sequence 
SND.WND - send window 
SND.BS - send buffer size 
SND.UP - send urgent pointer 
SND.WL - send sequence number used for last window update 
SND.LBB - send last buffer beginning 
ISS - initial send sequence number 
Receive Sequence Variables 
RCV.NXT - receive sequence 
RCV.WND - receive window 
RCV.BS - receive buffer size 
RCV. UP - receive urgent pointer 
RCV.LBB - receive last buffer beginning 
IRS  initial receive sequence number 
[Page 19] 
------------------------------<page break>-----------------------------
Transmission Control Protocol 
Functional Specification 
January 1980 
The following diagrams may help to relate some of these %'ariaDies to 
the sequence space. 
Send Sequence Space 
1 2 3 4 
SND. UNA SND.NXT SND. UNA 
+SND.WND 
1 - old sequence numbers which have been acknowledged 
2 - sequence numbers of unacknowledged data 
3 - sequence numbers allowed for new data transmission 
4 - future sequence numbers knich are not yet allowed 
Send S_aun Space 
Figure 
Receive Sequence Space 
1 2 3 
RCV. NXT RCV. NXT 
+RCV .WND 
1 - old sequence numbers which have been acknowledged 
2 - sequence numbers allowed for new reception 
3 - fuur squ_nc numbers which are not yet allowed 
Receive Sequence Space 
Figure 5. 
There are also some variables used frequently in the discussion that 
take their values from the fields of the current segment. 
[Page 20 ] 
------------------------------<page break>-----------------------------
January 1980 
Transmission Control Protocol 
Functional Specification 
Current Segment Variables 
SEG.SEQ - segment sequence number 
SEG.ACK - segment acknowledgment number 
SEG.LEN - segment length 
SEG.WND - segment window 
SEG.UP - segment urgent pointer 
SEG.PRC - segment precedence value 
A connection progresses through a series of states during its 
lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED, 
ESTABLISHED, FIN-WAIT-1, FiN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING, 
and the fictional state CLOSED. CLOSED is fictional because it 
represents the state when there is no TCB, and therefore, no 
connection. Briefly the meanings of the states are: 
LISTEN - represents waiting  for a connection request from any remote 
TCP and port. 
SYN-SENT - represents waiting for a matching connection request 
after having sent a connection request. 
SYN-RECEiVED - represents waiting for a confirming connection 
request acknowledgment after having both received and sent a 
connection request. 
ESTABLISHED - represents an open connection, ready to transmit and 
receive data segments. 
FIN-WAIT-1 - represents waiting for a connection termination request 
from the remote TCP, or an acknowledgment of the connection 
termination request previously sent. 
FIN-WAIT-2 - represents waiting for a connection termination request 
from the remote TCP. 
TIME-WAIT - represents waiting for enough time to pass to be sure 
the remote TCP received the acknowledgment of its connection 
termination request. 
CLOSE-WAIT - represents waiting for a connectior. termination request 
from the local user. 
CLOSING - represents waiting for a connection termination request 
acknowledgment from the remote TCP. 
CLOSED - represents no connection state at all. 
[Page 21 ] 
------------------------------<page break>-----------------------------
Transmission Control Protocol 
Functional Specification 
January 19B0 
A TCP connection progresses from one state to another in response to 
events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE, 
ABORT, and STATUS; the incoming segments, particularly those 
containing the SYN and FIN flags;-nd timeouts. 
The Glossary contains a more complete list of terms and their 
definitions. 
The state diagram in figure 6 only illustrates state changes, together 
with the causing events and resulting actions, but addresses neither 
error conditions nor actions which are not connected with state 
changes. In a later section, more detail is offered with respect to 
the reaction of the TCP to events. 
[Page 22] 
------------------------------<page break>-----------------------------
January 1980 
Transmission Control Protocol 
Functional Specification 
SYN 
RCVD 
, CLOSED 
passive OPEN : : CLOSE 
create TCB I I delete TCB 
V ' 
, LISTEN ' 
rcv SYN ' ' 
, , SEND 
snd SYN,ACX / \ snd SYN 
rcv SYN 
: x 
', CLOSE 
', snd FIN 
' CLOSE 
V 
 + snd FIN 
 FIN 
I WATT-1 
+ + rcv FIN 
, rcv ACK of FIN 
' snd ACK 
V x 
', F INWAIT-2 ', 
, rev FiN 
V snd ACK 
',TIME WAITI 
snd ACK 
rcv ACK of SYN \ / 
,, , 
v v 
' ESTAB 
Timeout=2MSL 
delete TCB 
rev SYN, ACK 
snd ACK 
I rcv FIN 
\ snd ACK 
/ CLOSE 
' snd FIN 
V V 
CLOSING I 
I rcv ACK of FIN 
V delete TCB 
CLOSED I 
TCP Connection State Diagram 
Figure 6. 
CLOSE 
active OPEN 
delete TCB I 
create TCB 
snd SYN 
\ \ 
, \ 
' V 
: SYN 
', SENT 
CLOSE 
WAIT 
[Page 23] 
------------------------------<page break>-----------------------------
Transmission Control Protocol 
Functional Specification 
January 1980 
3.3. Sequence Numbers 
A fundamental notion in the design is that every octet of data sent 
over a TCP connection has a sequence number. Since every octet is 
sequenced, each of them can be acknowledged. The acknowledgment 
mechanism employed is cumulative so that an acknowledgment of sequence 
number X indicates that all octets up to but not including X have been 
received. This mechanism allows for straight-forward duplicate 
detection in the presence of retransmission. Numbering of octets 
within a segment is that the first data octet immediately following 
the header is the lowest numbered, and the following octets are 
numbered consecutively. 
It is essential to remember that the actual sequence number space is 
finite, though very large. This space ranges from 0 to 232 - 1. 
Since the space is finite, all arithmetic dealing with sequence 
numbers must be performed modulo 2**32. This unsigned arithmetic 
preserves the relationship of sequence numbers as they cycle from 
2**32 - 1 to 0 again. There are some subtleties to computer modulo 
arithmetic, so great care should be taken in programming the 
comparison of such values. The typical kinds of sequence number 
comparisons which the TCP must perform include: 
(a) Determining that an acknowledgment refers to some sequence 
number sent but not yet acknowledged. 
(b) 
Determining that all sequence numbers occupied by a segment 
have been acknowledged (e.g., to remove the segment from a 
retransmission queue). 
(c) 
Determining that an incoming segment contains sequence numbers 
which are expected (i.e., that the segment "overlaps" the 
receive window). 
[Page 24 ] 
------------------------------<page break>-----------------------------
January 1980 
Transmission Control Protocol 
Functional Specification 
On send connections the following comparisons are needed: 
older sequence numbers _ newer sequence numbers 
SND.UNA SEG.ACK SND.NXT 
.... I .... XXXXXXX ...... XXXXXXXXXX ,XXXXXX .... 
I I I I I 
I 
Segment 1 Segment 2 Segment 3 
< ..... sequence space ..... > 
Sending Sequence Space Information 
Figure 7. 
SND.UNA = oldest unacknowledged sequence humocr 
SND.NXT = next sequence number to be sent 
SEG.ACK = acknowledgment (next sequence number expected by the 
acknowledging TCP) 
SEG.SEQ = first sequence number of a segment 
$EG.S=Q+,J.LEN-1 = last sequence number of a segment 
A new acknowledgment (called an "accept'able ack"), is one for which 
the inequality below holds: 
SND.UNA < SEG.ACX =< SND.NXT 
All arithmetic is modulo 2''32 and that comparisons are unsir, ed. 
"=<" means "less than or equal". 
A segment on the retransmission queue is fully acknowledged if the sum 
of its sequence number and length is less than the acknowledgment 
value in the incomin segment. 
SEG.LEN is the number of otets occupied by the data in the segment. 
It is important to note that SEG.LEN must be non-zero segments which 
do not occupy any sequence space (e.g., empty acknowledgment segments) 
are never placed on the retransmission queue, so would not go through 
this particular test. 
[ Page 25 ] 
------------------------------<page break>-----------------------------
Transmission Control Protocol 
Functional Specification 
January 1980 
On receive connections the following comparisons are needed: 
older sequence numbers newer sequence numbers 
RCV. NXT RCV.NXT+RCV.WND 
! 
XXXIXXX ...... XXXXXXXXXX XXXIXX 
Segment 1 Segment 2 Segment 3 
< .... sequence space ..... > 
Receiving Sequence Space Information 
Figure 8. 
RCV.NXT = next sequence number expected on incoming segments 
RCV.NXT+RCV.WND = last sequence number expected on incoming 
segments, plus one 
SEG.SEQ = first sequence number occupied by the incoming segment 
SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming 
segment 
A segment is judged to occupy a portion of valid receive sequence 
space if 
0 =< (SEG.SEQ+SEG.LEN-1 - RCV.NXT) < (RCV.NXT+RCV.WND - RCV.NXT) 
SEG.SEQ+SEG. LEN-1 is the last sequence number occupied by the segment; 
RCV.NXT is the next sequence number expected on an incoming segment; 
and RCV.NXT+RCV.WND is the right edge of the receive window. 
Actually, it is a little more complicated than this. Due to zero 
windows and zero length segments, we have four cases for the 
acceptability of an incoming segment: 
[Page 26] 
------------------------------<page break>-----------------------------
January 1980 
Transmission Control Protocol 
Functional Specification 
Segment Receive Test 
Length Window 
0 SEG.SEQ = RCV.NXT 
>0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND 
>0 0 not acceptable 
>0 
>0 RCV.NXT < SEG.SEQ+SEG.LEN =< RCV.NXT+RCV.WND 
Note that the acceptance test for a segment, since it requires the end 
of a segment to lie in the window, is somewhat more restrictive than 
is absolutely necessary. If at least the first sequence number of the 
segment lies in the receive window, or if some part of the segment 
lies in the receive window, then the segment might be judged 
acceptable. Thus, in figure 8, at least segments 1 and 2 are 
acceptable by the strict rule, and segment 3 may or may not be, 
depending on the strictness of interpretation of the rule. 
Note that when the receive window is zero no segments should be 
acceptable except ACK segments. Thus, it should be possible for a TCP 
to maintain a zero receive window while transmitting data and 
receiving ACKs. 
We have taken advantage of the numbering scheme to protect certain 
control information as well. This is achieved by implicitly including 
some control flags in the sequence space so they can be retransmitted 
and acknowledged without confusion (i.e., one and only one copy of the 
control will be acted upon). Control information is not physically 
carried in the segment data space. Consequently, we must adopt rules 
for implicitly assigning sequence numbers to control. The SYN and FIN 
are the only controls requiring this protection, and these controls 
are used only at connection opening and closing. For sequence number 
purposes, the S' is considered to occur before the first actual data 
octet of the segment in which it occurs, while the FIN is considered 
to occur after the last actual data octet in a segment in which it 
occurs. The segment length includes both data and sequence space 
occupying controls. When a SYN is present then SEG.SEQ is the 
sequence number of the SYN. 
Initial Sequence Number Selection 
The protocol places no restrioCion on a particular connection being 
used over and over again. A connection is defined by a pair of 
sockets. New instances of a connection will be referred to as 
incarnations of the connection. The problem that arises owing to this 
[Page 27] 
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Transmission Control Protocol 
Functional Specification 
January 1980 
is -- "how does the TCP identify duplicate segments from previous 
incarnations of the connection?" This problem becomes apparent if the 
connection is being opened and closed in quick succession, or if the 
connection breaks with loss of memoy and is then reestablished. 
To avoid confusion we must prevent segments from one incarnation of a 
connection from being used while the same sequence numbers may still 
be present in the network from an earlier incarnation. We want to 
assure this, even if a TCP crashes and loses all knowledge of the 
sequence numbers it has been using. When new connections are created, 
an initial sequence number (ISN) generator is employed which selects a 
new 32 bit ISN. The generator is bound to a (possibly fictitious) 32 
bit clock whose 1 order bit is incremented roughly every 4 
microseconds. Thus, the ISN cycles approximately every 4.55 hours. 
Since we assume that segments will stay in the network no more than 
tens of seconds or minutes, at worst, we can reasonably assume that 
ISN's will be unique. 
For each connection there is a send sequence number and a receive 
sequence number. The initial send sequence number (ISS) is chosen by 
the data sending TCP, and the initial receive sequence number (IRS) is 
learned during the connection establishing procedure. 
For a connection to be established or initialized, the two TCPs must 
synchronize on each other's initial sequence numbers. This is done in 
an exchange of connection establishing messages carrying a control bit 
called "$YN" (for synchronize) and the initial sequence numbers. As a 
shorthand, messages carrying the SYN bit are also called "SYNs". 
Hence, the solution requires a suitable mechanism for picking an 
initial sequence number and a slightly involved handshake to exchange 
the ISN's. A "three way handshake" is necessary because sequence 
numbers are not tied to a global clock in the network, and TCPs may 
have different mechanisms for picking the ISN's. The receiver of the 
first SYN has no way of knowing whether the segment was an old delayed 
one or not, unless it remembers the last sequence number used on the 
connection (which is not always possible), and so it must ask the 
sender to verify this SYN. 
The "three way handshake" and the advantages of a "clock-driven" 
scheme are discussed in [4]. 
Knowing When to Keep Quiet 
To be sure that a TCP does not create a segment that carries a 
sequence number which may be duplicated by an old segment remaining in 
the network, the TCP must keep quiet for a maximum segment lifetime 
(MSL) before assigning any sequence numbers upon starting up or 
recovering from a crash in which memory of sequence numbers in use was 
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January 1980 
Transmission Control Protocol 
Functional Specification 
lost. For this specification the MSL is taken to be 2 minutes. This 
is an engineering choice, and may be changed if experience indicates 
it is desirable to do so. Note that if a TCP is reinitialized in some 
sense, yet retains its memory of sequence numbers in use, then it need 
not wait at all; it must only be sure to use sequence numbers larger 
than those recently used. 
It should be noted that this strategy does not protect against 
spoofing or other replay type duplicate message problems. 
3.4. Establishing a connection 
The "three-way handshake" is the procedure used to establish a 
connection. This procedure normally is initiated by one TCP and 
responded to by another TCP. The procedure also works if two TCP 
simultaneously initiate the procedure. When simultaneous attempt 
occurs, the TCP receives a "SYN" segment which carries no 
acknowledgment after it has sent a "SYN". Of course, the arrival of 
an old duplicate "SYN" segment can potentially make it appear, to the 
recipient, that a simultaneous connection initiation is in progress. 
Proper use of "reset" segments can disambiguate these cases. Several 
examples of connection initiation follow. Although these examples do 
not show connection synchronization using data-carrying segments, this 
is perfectly legitimate, so long as the receiving TCP doesn't deliver 
the data to the user until it is clear the data is valid (i.e., the 
data must be buffered at the receiver until the connection reaches the 
ESTABLISHED state). The three-way handshake reduces the possibility 
of false connections. It is the implementation of a trade-off betwee 
memory and messages to provide information for this checking. 
The simplest three-way handshake is shown in figure 9 below. The 
figures should be interpreted in the following way. Each line is 
numbered for reference purposes. Right arrows (-->) indicate 
departure of a TCP segment from TCP A to TCP B, or arrival of a 
segment at B from A. Left arrows (<), indicate the reverse. 
Eilipsis (...) indicates a segment which is still in the network 
(delayed). An "Xi" indicates a segment which is lost or rejected. 
Comments appear in parentheses. TCP states represent the state AFTER 
the departure or arrival of the segment (whose contents are shown in 
the center of each line). Segment contents are shown in abbreviated 
form, with sequence number, control flags, and ACK field. Other 
fields such as window, addresses, lengths, and text have bee left out 
in the interest of clarity. 
[Page 29] 
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Transmission Control Protocol 
Functional Specification 
January 1980 
TCP A TCP B 
CLOSED LISTEN 
2. SYN-SENT --> <SEQ=100><CTL=SYN> 
--> SYN-RECEIVED 
3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 
4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> 
--> ESTABLISHED 
5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED 
Basic 3-Way Handshake for Connection Synchronization 
Figure 9. 
In line 2 of figure 9, TCP A begins by sending a SYN segment 
indicating that it will use sequence numbers starting with sequence 
number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it 
received from TCP A. Note that the acknowledgment field indicates TCP 
B is now expecting to hear sequence 101, acknowledging the SYN which 
occupied sequence 100. 
At line 4, TCP A responds with an empty segment containing an ACK for 
TCP B's SYN; and in line 5, TCP A sends some data. Note that the 
sequence number of the segment in line 5 is the same as in line 4 
because the ACK does not occupy sequence number space (if it did, we 
would wind up ACKing ACX's!). 
Simultaneous initiation is only slightly more complex, as is shown in 
figure 10. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to 
ESTABLISHED. 
The principle reason for the three-way handshake is to prevent old 
duplicate connection initiations from causing confusion. To deal with 
this, a special control message, reset, has been devised. If the 
receiving TCP is in a non-synchronized state (i.e., SYN-SENT, 
SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset. 
If the TCP is in one of the synchronized states (ESTABLISHED, 
FIN-WAIT-1, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING), it aborts the 
connection and informs its user. We discuss this latter case under 
"half-open" connections below. 
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January 1980 
Transmission Control Protocol 
Functional Specification 
4. 
5. 
6. 
7. 
TCP A 
CLOSED 
SYN-SENT --> <SEQ= 100><CTL =SYN> 
SYN-RECEIVED <-- <SEQ=300><CTL=SYN> 
... <SEQ= 100 ><CTL =SYN> 
TCP B 
CLOSED 
<-- SYN-SENT 
--> SYN-RECEIVED 
SYN-RECEIVED --> <SEQ=101><ACK=301><CTL=ACK> ... 
ESTABLISHED <-- <SEQ=301><ACK=101><CTL=ACK> <-- SYN-RECEIVED 
... <SEQ=101><ACK=301><CTL=ACX> --> ESTABLISHED 
Simultaneous Connection Synchronization 
Figure 10. 
TCP A 
1. CLOSED 
2. SYN-SENT --> <SEQ=100><CTL=SYN> 
3. (duplicate) ... <SEQ=1000><CTL=S'fN> 
4. SYN-SENT 
5. SYN-SENT 
TCP B 
LISTEN 
--> SYN-RECEiVED 
<-- <SEQ=300><ACK=1001><CTL=SYN,ACK> <-- SYN-RECEIVED 
--> <SEQ=1001><CTL=RST> --> LISTEN 
6 .... <SEQ= 100><CTL =S.YN> 
7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> 
8. ESTABLISHED--> <SEQ=101><ACK=401><CTL=ACK> 
Recovery from Old Duplicate SYN 
Figure 11. 
As a simple example of recovery from old duplicates, consider 
--> SYN-RECEIVED 
<-- SYN-RECEIVED 
--> ESTABLISHED 
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Transmission CoNtrol Protocol 
Functional Specification 
January 1980 
figure 11. At line 3, an old duplicate SYN arrives at TCP B. TCP B 
cannot tell that this is an old duplicate, so it responds normally 
(line 4). TCP A detects that the ACK field is incorrect and returns a 
EST (reset) with its SEQ field selected to make the segment 
believable. TCP B, on receiving the ST, returns to the LISTEN state. 
ken the original SYN (pun intended) finally arrives at line 6, the 
synchronization proceeds normally. If the SYN at line 6 had arrived 
before the EST, a more complex exchange might have occurred with EST's 
sent in both directions. 
Half-Open Connections and Other Anomalies 
An established connection is said to be "half-open" if one of the 
TCPs has closed or aborted the connection at its end without the 
knowledge of the other, or if the two ends of the connection have 
become desynchronized owing to a crash that resulted in loss of 
memory. Such connections will automatically become reset if an 
attempt is made to send data in either direction. However, half-open 
connections are expected to be unusual, and the recovery procedure is 
mildly involved. 
If at site A the connection no longer exists, then an attempt by the 
user at site B to send any data on it will result in the site B TCP 
receiving a reset control message. Such a message should indicate to 
the site B TCP that something is wrong, and it is expected to abort 
the connection. 
Assume that two user processes A and B are communicating with one 
another when a crash occurs causing loss of memory to A's TCP. 
Depending on the operating system supporting A's TCP, it is likely 
that some error recovery mechanism exists. nen the TCP is up again, 
A is likely to start again from the beginning or from a recovery 
point. As a result, A will probably try to OPEN the connection again 
or try to SEND on the connection it believes open. In the latter 
case, it receives the error message "connection not open" from the 
local (A's) TCP. In an attempt to establish the connection, A's TCP 
will send a segment containing SYN. This scenario leads to the 
example shown in figure 12. After TCP A crashes, the user attempts to 
re-open the connection. TCP B, in the meantime, thinks the connection 
is ope. 
[Page 32 ] 
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January 1980 
Transmission Control Protocol 
Functional Specification 
TCP A - TCP B 
1. (CRASH) 
(send 300,receive 100) 
2. CLOSED 
ESTABLISHED 
3. SYN-SENT --> <SEQ=400><CTL=SYN> 
--> (??) 
<-- <SEQ=300> <ACK= 100> <CTL =ACK> 
<-- ESTABLISHED 
5. SYN-SENT --> <SEQ=100><C=RST> 
--> (Abort!!) 
6 
7. SYN-SENT--> <SEQ=4CO><CTL=SYN> 
--> 
Half-Open Connection Discovery 
Figure 12. 
When the S' arrives at line 3, TCP B, being in a synchronized state, 
responds with an acknowledgment indicating what sequence it next 
expects to hear (ACK 100). TCP A sees that this segment does not 
acknowledge anything it sent and, being unsynchronized, sends a reset 
(RST) Decause it has detected a half-open connection. TC? B abcrts at 
line 5. TCP A will continue to try to establish the connection; the 
problem is now reduced to the basic 3ay handshake of figure 9. 
An interesting alternative case occurs when TCP A crashes and TCP B 
tries to send data on what it thinks is a synchronized connection. 
This is illustrated in figure 13. In this case, the data arriving at 
TCP A from TCP B (line 2) is unacceptable because no such connection 
exists, so TCP A sends a RST. The RST is acceptable so TCP B 
processes it and aborts the connection. 
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Transmission Control Protocol 
Functional Specification 
January 1980 
TCP A -- TCP B 
1. (CRASH) (send 3OO,receive 100) 
2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED 
3. --> <SEQ=100><CTL=RST> --> (ABORT!!) 
Active Side Causes Half-Open Connection Discovery 
Figure 13. 
In figure 14, we find the two TCPs A and B with passive connections 
waiting for SYN. An old duplicate arriving at TCP B (line 2) stirs B 
into action. A SYN-ACK is returned (line 3) and causes TCP A to 
generate a RST (the ACK in line 3 is not acceptable). TCP B accepts 
the reset and returns to its passive LISTEN state. 
TCP A 
1. LISTEN 
2 .... <SEQ=Z ><CTL =SYN> 
3. (??) <-- <SEQ=X><ACK=Z+I><CTL=SYN,ACK> 
4. --> <SEQ=Z+I><CTL=RST> 
5. LISTEN 
--> 
TCP B 
LISTEN 
SYN-RECEiVED 
SYN-RECEIVED 
(return to LISTEN!) 
LISTEN 
Old Duplicate SYN Initiates a Reset on two Passive Sockets 
Figure 14. 
A variety of other cases are possible, all of which are accounted for 
by the following rules for RST generation and processing. 
Reset Generation 
As a general rule, reset (RST) should be sent whenever a segment 
arrives which apparently is not intended for the current or a future 
incarnation of the connection. A reset should not be sent if it is 
not clear that this is the case. Thus, if any segment arrives for a 
nonexistent connection, a reset should be sent. If a segment ACKs 
[Page 34] 
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January 1980 
Transmission Control Protocol 
Functional Specification 
something which has never been sent on the current connection, then 
one of the following two cases applies. 
1. If the connection is in any non-synchronized state (hISTEN, 
SYN-SENT, SYN-RECEiVED) or if the connection does not ex'ist, a reset 
(RST) should be formed and sent for any segment that acknowledges 
something not yet sent. The RST should take its SEQ field from the 
ACK field of the offending segment (if the ACK control bit was set), 
and its ACK bit should be reset (zero), except to refuse a initial 
SYN. A reset is also sent if an incoming segment has a security level 
or compartment which does not exactly match the level and compartment 
requested for the connection. If the precedence of the incoming. 
segment is less than the precedence level requested a reset is sent. 
2. If the connection is in a synchronized state (ESTABLISHED, 
FINdWAIT-I, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING), any 
unacceptable segment should elicit only an empty acknowledgment 
segment containing the current send-sequence number and an 
acknowledgment indicating the next sequence number expected to be 
received. 
Reset Processing 
All reset (RST) segments are validated by checking their SEQ-fields. 
A reset is valid if its sequence number is in the window. In the case 
of a RST received in response to an initial SYN any sequence number is 
acceptable if the ACK field acknowledges the SYN. 
The receiver of a RST first validates it, then changes state. If the 
receiver was in the LISTEN state, it ignores it. If the receiver was 
in SYN-RECEiVED state and had previously been in the LISTEN state, 
then the receiver returns to the LISTEN state, otherwise the receiver 
aborts the connection and goes to the CLOSED state. If the receiver 
was in any other state, it aborts the connection and advises the user 
and goes to the CLOSED state. 
3.5. Closing a Connection 
CLOSE is an operation meaning "I have no more data to send." The 
notion of closing a full-duplex connection is subject to ambiguous 
interpretation, of course, since it may not be obvious how to treat 
the receiving side of the connection. We have chosen to treat CLOSE 
in a simplex fashion. The user who CLOSEs may continue to RECEIVE 
until he is told that the other side has CLOSED also. Thus, a program 
could initiate several SENDs followed by a CLOSE, and then continue to 
RECEIVE until signaled that a RECEIVE failed because the other side 
has CLOSED. We assume that the TCP will signal a user, even if no 
RECEIVEs are outstanding, that the other side has closed, so the user 
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Transmission Control Protocol 
Functional Specification 
January 1980 
can terminate his side gracefully. A TCP will reliably deliver all 
buffers SENT before the connection was CLOSED so a user who expects no 
data in return need only wait to har the connection was CLOSED 
successfully to know that all his data was received at the destination 
TCP. 
There are essentially three cases: 
1) The user initiates by telling the TCP to CLOSE the connection 
2) The remote TCP initiates by sending a FIN control signal 
3) Both users CLOSE simultaneously 
Case 1: Local user initiates the close 
In this case, a FIN segment can be constructed and placed on the 
outgoing segment queue. No further SENDs from the user will be 
accepted by the TCP, and it enters the FINA!T-1 state. RECEIVEs 
are allowed in this state. All segments preceding and including FiN 
will be retransmitted until acknowledged. When the other TCP has 
both acknowledged the FiN and sent a FiN of its own, the first TCP 
can ACK this FIN. It should be noted that a TCP receiving a FIN 
will ACK but not send its own FIN until its user has CLOSED the 
connection also. 
Case 2: TCP receives a FIN from the network 
If an unsolicited FIN arrives from the network, the receiving TCP 
car, ACK it and tell the user that the connection is closing. The 
user should respond with a CLOSE, upon which the TCP can send a FIN 
to the other TCP. The TCP then waits until its own FiN is 
acknowledged whereupon it deletes the connection. If an ACK is not 
forthcoming, after a timeout the connection is aborted and the user 
is told. 
Case 3: both users close simultaneously 
A simultaneous CLOSE by users at both ends of a connection causes 
FIN segments to be exchanged. When all segments preceding the FINs 
have been processed and acknowledged, each TCP can ACE the FIN it 
has received. Both will, upon receiving these ACKs, delete the 
connection. 
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January 1980 
Transmission Control Protocol 
Functional Specification 
Be 
TCP A 
ESTABLISHED 
(Close) 
F IN-WAIT-1 
FIN-WAIT-2 
TIME-WAIT 
TIME-WAIT 
(2 MSL) 
CLOSED 
--> <SEQ=100><CTL=FIN> 
<-- <SEQ=300><ACK=101><CTL=ACK> 
<-- <SEQ=301><CTL:FIN> 
--> <SEQ: 100> <ACK=30 I><CTL =ACK> 
Normal Close Sequence 
Figure 15. 
TCP B 
ESTABLISHED 
--> CLOSE-WAIT 
<-- CLOSEAIT 
(Close) 
<-- CLOSING 
--> CLOSED 
Be 
0 
TCP A 
ESTABLISHED 
(Close) 
FIN-WAIT-1 
CLOSING 
CLOSED 
--> <SEQ=100><CTL =FIN> 
<-- <SEQ=300><CTL =FIN> 
... <SEQ=100><CTL =FIN> 
--> <SEQ=100><ACK=301><CTL=ACK> 
<-- <SEQ=3OO><ACK=101><CTL=ACK> 
 .. <SEQ=100><ACK=301><CTL=ACK> 
Simultaneous Close Sequence 
Figure 16. 
TCP B 
ESTABLISHED 
(Close) 
... FIN-WAIT-1 
--> 
... CLOSING 
CLOSED 
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Transmission Control Protocol 
Functional Specification 
January 1980 
3.6. Precedence and Security 
The intent is that connection be allowed only between ports operating 
with exactly the same security and compartment values and at the 
higher of the precedence level requested by the two parts. 
The precedence levels are: 
flash override - 111 
flash - 110 
immediate - 10X 
priority - 01X 
routine - O0X 
The security levels are: 
top secret - 11 
secret - 10 
confidential - 01 
unclassified - O0 
The compartments are assigned by the Defense Communications Agency. 
The defaults are precedence: routine, security: unclassified, 
compartment: zero. A host which does not implement precedence or 
security feature should clear these fields to zero for segments it 
sends. 
A connection attempt with mismatched security/compartment values or a 
lower precedence value should be rejected by sending a reset. 
Note that TCP modules which operate only at the default value of 
precedence will still have to check the precedence of incoming 
segments and possibly raise the precedence level they use on the 
connection. 
3.7. Data Communication 
Once the connection is established data is communicated by the 
exchange of segments. Because segments may be lost due to errors 
(checksum test failure), or network congestion, TCP uses 
retransmission (after a timeout) to ensure delivery of every segment. 
Duplicate segments may arrive due to network or TCP retransmission. 
As discussed in the section on sequence numbers the TCP performs 
certain tests on the sequence and acknowledgment numbers in the 
segments to verify their acceptability. 
The sender of data keeps track of the next sequence number to use in 
the variable SND.NXT. The receiver of data keeps track of the next 
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January 1980 
Transmission Control Protocol 
Functional Specification 
sequence number to expect in the variable RCV.NXT. The sender of data 
keeps track of the oldest unacknowledged sequence number in the 
variable SND.UNA. If the data flow is momentarily idle and all data 
sent has been acknowledged then the three variables will be equal. 
When the sender creates a segment and transmits it the sender advances 
SND.NXT. When the receiver accepts a segment it advances RCV.NX and 
sends an acknowledgment. When the data sender receives an 
acknowledgment it advances SND.UNA. The extent to which the values of 
these variables differ is a measure of the delay in the communication. 
Normally the amount by which the variables are advanced is the length 
of the data in the segment. However, when letters are used there are 
special provisions for coordination the sequence numbers, the letter 
boundaries, and the receive buffer boundaries. 
End of Letter Sequence Number Adjustments 
There is provision in TCP for the receiver of data to optionally 
communicate to the sender of data on a connection at the time of the 
connection synchronization the receiver's buffer size. If this is 
done the receiver must use this fixed size of buffers for the lifetime 
of the connection. If a buffer size is communicated then there is a 
coordination between receive buffers, letters, and sequence numbers. 
Each time a buffer is completed either due to being filled or due to 
an end of letter, the sequence number is incremented through the end 
of that buffer. 
That is, whenever an EOL is transmitted, the sender advances its send 
sequence number, SND.NXT, by an amount sufficient to consume all the 
unused space in the receiver's buffer. The amount of space consumed 
in this fashion is subtracted from the send window just as is the 
space consumed by actual data. 
And, whenever an EOL is received, the receiver advances its receive 
sequence number, RCV. NXT, by an amount sufficient to consume all the 
unused space in the receiver's buffer. The amount of space consumed 
in this fashion is subtracted from the receive window just as is the 
space consumed by actual data. 
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Transmission Control Protocol 
Functional Specification 
January 1980 
older sequence numbers 
newer sequence numbers 
Buffer 1 
i 
Buffer 2 
XXXXXXXXXXX,XX++++++++++ 
I< ..... SEG.LEN ...... >I I 
i i i 
i i 
SEG.SEQ A B 
XXX - data octeta from segment 
+++ - phantom data 
< ..... sequence space ...... > 
End of Letter Adjustment 
Figure 17. 
In the case illustrated above, if the segment does not carry an EOL 
flag, the next value of SND.NXT or RCV.NXT will be A. If it does 
carry an EOL flag, the next value will be B. 
/]ae exchange of buffer size and sequencing information is done in 
units of octeta. If no buffer size is stated, then the buffer size is 
assumed to be 1 octet. The receiver tells the sender the size of the 
buffer in a SYN segment that contains the 16 bit buffer size data in 
an option field in the TCP header. 
Each EOL advances the sequence number (SN) to the next buffer boundary 
While LBB < SEG.SEQ+SEG.LEN 
Do LBB <- LBB + B$ End 
SN <- LBB 
where LBB is the Last Buffer Beginning, and BS is the buffer size. 
The CLOSE user call implies an end of letter, as does the FIN control 
flag in an incoming segment. 
The Communication of Urgent Information 
The objective of the TCP urgent mechanism is to allow the sending user 
to stimulate the receiving user to accept some urgent data and to 
permit the receiving TCP to indicate to the receiving user when all 
the currently known urgent data has been received by the user. 
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January 1980 
Transmission Control Protocol 
Functional Soecification 
This mechanism permits a point in the data stream to be designated as 
the end of "urgent" information. Whenever this point is in advance of 
the receive sequence humDer (RCV.NXT) at the receiving TCP, that TCP 
should tell the user to go into "ugent mode"; when the receive 
sequence number catches up to the urgent pointer, the TCP should tell 
user to go into "normal mode". If the urgent pointer is updated while 
the user is in "read fast" mode, the update will be invisible to the 
The method employs a urgent field which is carried in all segments 
transmitted. The URG control flag indicates that the urgent field is 
meaningful and should be added to the segment sequence number to yield 
the urgent pointer. The absence of this flag indicates that the 
urgent pointer has not changed. 
To send an urgent indication the user must also send at least one data 
octet. If the sending user.also indicates end of letter, timely 
delivery of the urgent information to the destination process is 
enhanced. 
Managing the Window 
The window sent in each segment indicates the range of sequence number 
the sender of the window (the data receiver) is currently prepared to 
accept. There is an assumption that this is related to the currently 
available data buffer space available for this connection. The window 
information is a guideline to be aimed at. 
Indicating a large window encourages transmissions. If more data 
arrives than can be accepted, it will be discarded. This will result 
in excessive retransmissions, adding unnecessarily to the load on the 
network and the TCPs. Indicating a small window may restrict the 
transmission of data to the Doint of introducing a round trip delay 
between each new segment transmitted. 
The mechanisms provided allow a TCP to advertise a large window and to 
subsequently advertise a much smaller window without having accepted 
that much data. This, so called "shrinking the window," is strongly 
discouraged. The robustness principle dictates that TCPs will not 
shrink the window themselves, but will be prepared for such behavior 
on the part of other TCPs. 
The sending TCP must be prepared to accept and send at least one octet 
of new data even if the send window is zero. The sending TCP should 
regularly retransmit to the receiving TCP even when the window is 
zero. Two minutes is recommended for the retransmission interval when 
the window is zero. This retransmission is essential to guarantee 
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Transmission Control Protocol 
Functional Specification 
January 1980 
that when either TCP has a zero window the re-opening of the window 
will be reliably reported to the other. 
The sending TCP packages the data to-be transmitted into segments 
which fit the current window, and may repackage segments on the 
retransmission queue. Such repackaging is not required, but may be 
helpful. 
Users must keep reading connections they close for sending until the 
TCP says no more data. 
In a connection with a one-way data flow, the window information will 
be carried in acknowledgment segments that all have the same sequence 
number so there will be no way to reorder them if they arrive out of 
order. This is not a serious problem, but it wi!l allow the window 
information to be on occasion temporarily based on old reports from 
the data receiver. 
3.8. Interfaces 
There are of course two interfaces of concern: the user/TCP interface 
and the TCP/IP interface. We have a fairly elaborate model of the 
user/TCP interface, but only a sketch of the interface to the lower 
level protocol module. 
User/TCP Interface 
The functional description of user commands to the TCP is, at best, 
fictional, since every operating system will have different 
facilities. Consequently, we must warn readers that different TCP 
implementations may have different user interfaces. However, all 
TCPs must provide a certain minimum set of services to guarantee 
that all TCP implementations can support the same protocol 
hierarchy. This section specifies the functional interfaces 
required of all TCP implementations. 
TCP User Commands 
The following sections functionally characterize a USER/TCP 
interface. The notation used is similar to most procedure or 
function calls in high level languages, but this usage is not 
meant to rule out trap type service calls (e.g., SVCs, UUOs, 
EMTs). 
The user commands described below specify the basic functions the 
TCP must perform to support interprocess communication. 
Individual implementations should define their own exact format, 
and may provide combinations or subsets of the basic functions in 
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January 1980 
Transmission Control Protocol 
Fuhctional Specification 
single calls. In particular, some implementations may wish to 
automatically OPEN a connection on the first SEND or RECEIVE 
issued by the user for a given connection. 
In providing interprocess communication facilities, the TCP must 
not only accept commands, but must also return information to the 
processes it serves. The latter consists of: 
(a) general information about a connection (e.g., interrupts, 
remote close, binding of unspecified foreign socket). 
(b) replies to specific user commands indicating success or 
various types of failure. 
Open 
Format: OPEN (local port, foreign socket, active/passive 
[, buffer size] [, timeout] [, precedence] 
[, security/compartment]) -> local connection name 
We assume that the local TCP is aware of the identity of the 
processes it serves and will check the authority of the process 
to use the connection specified. Depending upon the 
implementation of the TCP, the local network and TCP identifiers 
for the source address will either be supplied by the TCP or by 
the processes that serve it (e.g., the program which interfaces 
the TCP network). These considerations are the result of 
concern about security, to the extent that no TCP be able to 
masquerade as another one, and so on. Similarly, no process can 
masquerade as another without the collusion of the TCP. 
If the active/passive flag is set to passive, then this is a 
call to LISTEN for an incoming connection. A passive open may 
have either a fully specified foreign socket to wait for a 
particular connection or an unspecified foreign socket to wait 
for any call. A fully specified passive call can be made active 
by the subsequent execution of a SEND. 
A full-duplex transmission dontrol block (TCB) is created and 
partially filled in with data from the OPEN command parameters. 
On an active OPEN command, the TCP will begin the procedure to 
synchronize (i.e., establish) the connection at once. 
The buffer size, if present, indicates that the caller will 
always receive data from the connection in that size of buffers. 
This buffer size is a measure of the buffer between the user and 
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